This application claims benefit of Japanese Patent Application No. 2001-122835 filed in Japan on Apr. 20, 2001, the contents of which are incorporated by this reference.
The present invention relates to a viewing optical system and an image pickup optical system and also pertains to an apparatus using the viewing optical system and/or the image pickup optical system. More particularly, the present invention relates to an optical system for use in an image display apparatus or the like that can be retained on an observer""s head or face and can also be added to a portable telephone or a portable information terminal.
In recent years, image display apparatus, particularly head- or face-mounted image display apparatus, have been developed actively for the purpose of enabling the user to enjoy viewing wide-screen images personally. Meanwhile, portable telephones have recently become widespread, and there have been increasing demands that portable information terminals should display images and character data on a large screen.
Under these circumstances, Japanese Patent Application Unexamined Publication Numbers [hereinafter referred to as xe2x80x9cJP(A)xe2x80x9d] Hei 7-140414 and Hei 9-171151 propose an optical system using a half-mirror as an oblique mirror for branching an optical path in a prism optical system including a concave mirror having a small amount of decentration.
U.S. Pat. No. 5,093,567 and JP(A) 2000-241751 and 2000-180787 propose an optical system in which a first prism having a triangular configuration and a convex lens action is disposed on the eye side of the optical system, and a second prism is disposed to face the first prism across a small air space. These conventional techniques propose a viewing optical system that folds an optical path without loss of light quantity by making use of a total reflection phenomenon occurring owing to the refractive index difference between glass and air produced by the presence of the small air space between the two prisms.
U.S. Pat. No. 4,874,214 proposes a viewing optical system using a holographic element. In this viewing optical system, holographic elements are used at two places, i.e. on an oblique mirror surface that is a plane surface, and on a spherical substrate surface.
In the above-mentioned JP(A) Hei 7-140414 and Hei 9-171151, an oblique mirror disposed in a prism optical system is formed from a half-mirror. With this arrangement, light emitted from an image display device passes through the half-mirror twice. Therefore, the amount of light reduces to xc2xc, and thus the displayed image becomes unfavorably dark. In order to prevent this problem, it is necessary to illuminate the image display device by using bright illumination or the like consuming more electric power. In a case where the luminance of the light source cannot be increased owing to power consumption or the capability of the light source device, it becomes impossible to view the displayed image under the bright sun.
In the viewing optical systems proposed in U.S. Pat. No. 5,093,567 and JP(A) 2000-241751 and 2000-180787, it is necessary to adjust the optical axes of the two prisms with respect to each other because a small air space is provided between the prisms. Therefore, the assembly cost increases. Further, it is likely that the optical axes of the two prisms will be displaced from each other when an impact or vibration is applied to an apparatus including the viewing optical system.
The optical system proposed in U.S. Pat. No. 5,093,567 is a relay optical system and hence large in size and heavy in weight. Therefore, it is difficult to use the optical system in a portable telephone or a portable information terminal.
The viewing optical system proposed in U.S. Pat. No. 4,874,214 has a spherical holographic element on a spherical surface. Incidentally, a holographic element has two different kinds of optical power, i.e. an optical power derived from a geometrical configuration, and an optical power based on the diffractive effect of the holographic element. Two different kinds of power obtained when a holographic element is provided on a substrate member having a spherical surface, for example, will be explained below with reference to FIGS. 29(a) and 29(b). As shown in FIG. 29(a), the holographic element has a power based on the difference in density of interference fringes, e.g. the pitch of periodic structures in the holographic element. In addition, the holographic element has an optical power derived from the geometrical configuration thereof, as shown in FIG. 29(b). Regarding the optical power based on the geometrical configuration, the optical powers "PHgr" of a conventional optical refractive lens and conventional reflecting mirrors can be calculated according to the following equations:
Refracting system: "PHgr"=(nxe2x88x921)(1/R)
Surface-coated mirror: "PHgr"=2/R
Back-coated mirror: "PHgr"=2n/R
where "PHgr": the optical power based on the geometrical configuration
n: the refractive index of the medium
R: the radius of curvature of the hologram substrate
Accordingly, it will be understood from a comparison of the surface-coated mirror with the back-coated mirror that the back-coated mirror can obtain a given optical power based on the geometrical configuration with a gentler curvature (larger curvature radius R) by 1/n than in the case of the surface-coated mirror.
That is, even if the geometrical configuration of a reflection type holographic element is formed with a gentle curvature (large curvature radius R), it is possible to obtain a large optical power based on the geometrical configuration by filling the inside of the holographic element with a medium having a refractive index n, e.g. a glass or plastic material, as in the case of the back-coated mirror.
Thus, aberrations occurring at the hologram surface can be suppressed by employing an arrangement that allows a large optical power to be produced with a gentle curvature (large curvature radius R) in the optical system.
However, in the viewing optical system stated in the above-mentioned U.S. Pat. No. 4,874,214, the space between the plane surface and the spherical surface is not filled with a glass or plastic medium. Therefore, it is necessary to form the geometrical configuration with a reduced curvature radius R in order to ensure the required optical power derived from the geometrical configuration having a spherical shape.
When the geometrical configuration is formed with a reduced curvature radius R, aberrations occurring at this reflecting surface increase, and it becomes difficult to effect favorable image display. Further, because there is no optical surface in the optical path between the image plane and the above-described plane surface, it is difficult to correct distortion favorably.
Further, the hologram surface in U.S. Pat. No. 4,874,214 is a spherical surface. In general, methods of bonding a hologram are divided into-one type in which a film-shaped hologram is bonded to a substrate surface, and another type in which a substrate surface is sprayed with a liquid photopolymer or the like as a hologram recording material. The latter method needs to carry out exposure and development after the spraying process. Considering mass-productivity, it is preferable to adopt the method wherein a film-shaped holographic element is bonded to a substrate because this method allows exposure and development to be performed before the holographic element is bonded to the substrate.
However, film-shaped holograms supplied from manufacturers are, in general, plane holograms. It is not easy to bond a film-type holographic element on a three-dimensional curved surface uniformly.
The present invention was made to solve the above-described problems with the prior art.
An object of the present invention is to provide a viewing optical system for image display apparatus that allows observation of a bright displayed image favorably corrected for aberrations and is easy to assemble, resistant to impact such as vibration, lightweight and compact, and also provide an image pickup optical system and an apparatus using the viewing optical system and/or the image pickup optical system.
To attain the above-described object, the present invention provides a viewing optical system having an observation image forming member for forming an observation image to be viewed by an observer and an ocular optical member for leading the observation image formed by the observation image forming member to an exit pupil formed at the position of an observer""s eyeball.
The ocular optical member includes at least a first prism member and a second prism member.
The first prism member has, at least, a first entrance surface through which light rays from the observation image enter the first prism member, a reflecting surface reflecting the light rays within the first prism member, and a first exit surface through which the light rays exit the first prism member. The first entrance surface, the reflecting surface and the first exit surface are disposed to face each other across a first prism medium.
The second prism member has, at least, a second entrance surface through which the light rays exiting from the first prism member enter the second prism member, and a second exit surface through which the light rays exit the second prism member. The second entrance surface and the second exit surface are disposed to face each other across a second prism medium.
The first prism member and the second prism member are cemented together with a holographic element interposed between the first exit surface and the second entrance surface.
The reflecting surface of the first prism member is a concave surface that gives a positive power to the light rays when reflecting them.
The first exit surface and the second entrance surface are each formed from a plane surface or a cylindrical surface. Alternatively, the first exit surface and the second entrance surface are each formed from a spherical surface or a toric surface satisfying the following conditions:
xe2x88x922.0 less than Da/Ra less than 2.0xe2x80x83xe2x80x83(2)
xe2x88x920.05 less than Db/Rb less than 0.05xe2x80x83xe2x80x83(3)
where Ra and Da are a curvature radius and an outer diameter of the surface in the direction of an axis where the surface has a larger curvature, and Rb and Db are a curvature radius and an outer diameter of the surface in the direction of an axis where the surface has a smaller curvature.
In addition, the present invention provides an image pickup optical system having an image pickup device placed in an image plane to pick up an image of an object, an aperture stop placed in a pupil plane to reduce the brightness of a light beam from the object, and an image-forming optical member disposed between the image plane and the pupil plane to lead the object image to the image plane.
The image-forming optical member includes at least a second prism member and a first prism member.
The second prism member has, at least, a third entrance surface through which light rays emanating from the object and passing through the aperture stop enter the second prism member, and a third exit surface through which the light rays exit the second prism member. The third entrance surface and the third exit surface are disposed to face each other across a second prism medium.
The first prism member has, at least, a fourth entrance surface through which the light rays exiting from the second prism member enter the first prism member, a reflecting surface reflecting the light rays within the first prism member, and a fourth exit surface through which the light rays exit the first prism member. The fourth entrance surface, the reflecting surface and the fourth exit surface are disposed to face each other across a first prism medium.
The second prism member and the first prism member are cemented together with a holographic element interposed between the third exit surface and the fourth entrance surface.
The reflecting surface of the first prism member is a concave surface that gives a positive power to the light rays when reflecting them.
The third exit surface and the fourth entrance surface are each formed from a plane surface or a cylindrical surface. Alternatively, the third exit surface and the fourth entrance surface are each formed from a spherical surface or a toric surface satisfying the following conditions:
xe2x88x922.0 less than Da/Ra less than 2.0xe2x80x83xe2x80x83(2)
xe2x88x920.05 less than Db/Rb less than 0.05xe2x80x83xe2x80x83(3)
where Ra and Da are a curvature radius and an outer diameter of the surface in the direction of an axis where the surface has a larger curvature, and Rb and Db are a curvature radius and an outer diameter of the surface in the direction of an axis where the surface has a smaller curvature.
The reasons for adopting the above-described arrangements in the present invention, together with the functions thereof, will be described below.
First, the viewing optical system will be described.
The viewing optical system according to the present invention has an observation image forming member for forming an observation image to be viewed by an observer and an ocular optical member for leading the observation image formed by the observation image forming member to an exit pupil formed at the position of an observer""s eyeball. The ocular optical member includes at least a first prism member and a second prism member. The first prism member has, at least, a first entrance surface through which light rays from the observation image enter the first prism member, a reflecting surface reflecting the light rays within the first prism member, and a first exit surface through which the light rays exit the first prism member. The first entrance surface, the reflecting surface and the first exit surface are disposed to face each other across a first prism medium. The second prism member has, at least, a second entrance surface through which the light rays exiting from the first prism member enter the second prism member, and a second exit surface through which the light rays exit the second prism member. The second entrance surface and the second exit surface are disposed to face each other across a second prism medium.
Thus, the inside of the ocular optical member is filled with a medium, e.g. a glass or plastic material, thereby increasing the optical power based on the surface configuration of each optical functional surface, and thus favorably correcting aberrations, e.g. spherical aberration and comatic aberration.
Further, in the above-described viewing optical system according to the present invention, the first prism member and the second prism member are cemented together with a holographic element interposed between the first exit surface and the second entrance surface.
If a holographic element is used as an oblique mirror for branching an optical path, a diffraction efficiency close to 100% can be obtained when light rays are reflected and diffracted. Thus, it becomes possible to display a bright image without loss of light quantity. If the two prism members, i.e. the prism member closer to the image display device (observation image forming member), and the eye-side prism member, are cemented together into a single member with a holographic element interposed therebetween, it is possible to solve such problems as optical axis displacement that may occur during assembly owing to the presence of an air space, and troublesomeness in setting. Thus, it is possible to attain a viewing optical system easy to assemble and resistant to impact such as vibration.
Further, if the holographic element is cemented between the first prism member and the second prism member, it is possible to keep the holographic element out of dust and hence possible to prevent dust or other foreign matter from being undesirably observed as an enlarged image without the need to provide a dustproof member separately. It is also possible to prevent water from entering the holographic element from the outside, which would otherwise swell the holographic element, causing a change in the peak wavelength of the diffraction efficiency.
Further, in the above-described viewing optical system according to the present invention, the reflecting surface of the first prism member is a concave surface that gives a positive power to the light rays when reflecting them.
Further, the viewing optical system according to the present invention does not form an intermediate image between the image display device and the observer""s eye. That is, the viewing optical system has no relay optical system. Therefore, it is constructed in the form of a lightweight and compact viewing optical system.
Further, it is desirable in the viewing optical system according to the present invention that the first entrance surface of the first prism member should have a curved surface configuration that gives a power to light rays when they pass through the surface, and the second exit surface of the second prism member should have a curved surface configuration that gives a power to light rays when they pass through the surface.
Further, it is preferable in the viewing optical system according to the present invention that the first prism medium and the second prism medium should be the same kind of medium.
Further, it is preferable in the viewing optical system according to the present invention that the first exit surface of the first prism member and the second entrance surface of the second prism member should have approximately the same surface configuration.
It should be noted that the term xe2x80x9capproximately the same surface configurationxe2x80x9d as used herein means that a difference in surface configuration within the range of manufacturing errors is permitted.
Further, it is preferable in the viewing optical system according to the present invention that a ghost light eliminating member that prevents ghost light from entering the observer""s eyeball should be provided on a non-optical functional surface other than the optical functional surfaces of the first and second prism members that transmit or reflect light rays.
The ghost light eliminating member is particularly effective when provided on the bottom and side surfaces of the ocular optical member when the first entrance surface of the first prism member is defined as the top surface. The term xe2x80x9cnon-optical functional surfacesxe2x80x9d includes the region outside the ray effective diameter of the first entrance surface, the region outside the ray effective diameter of the reflecting surface of the first prism member, and the region outside the ray effective diameter of the second exit surface of the second prism member. Providing a ghost light eliminating member on each of these regions is also effective.
Further, it is preferable in the viewing optical system according to the present invention that the first entrance surface of the first prism member should have a rotationally asymmetric curved surface configuration.
If a transmitting surface (i.e. the first entrance surface of the first prism member) is disposed in front of an image forming member, e.g. an image display device, as in the present invention, distortion can be corrected favorably. It should be noted that the surface in front of the image forming member may be a rotationally symmetric surface. However, it is even more desirable to use a free-form surface from the viewpoint of correcting decentration aberrations occurring when optical functional surfaces are decentered for the purpose of minimizing the size of the viewing optical system.
Further, it is preferable in the viewing optical system according to the present invention that the rotationally asymmetric curved surface configuration of the first entrance surface of the first prism member should be a free-form surface having only one plane of symmetry, and the plane of symmetry of the free-form surface should be coincident with a plane (YZ-plane) in which the optical axis is folded.
Further, it is preferable in the viewing optical system according to the present invention that the holographic element should be arranged to correct light rays for both rotationally symmetric and rotationally asymmetric components of lateral chromatic aberration by reflection and diffraction.
If the rotationally symmetric and rotationally asymmetric components of lateral chromatic aberration are corrected by a reflection type holographic element, a high contrast can be realized.
In the viewing optical system according to the present invention, the holographic element cemented between the first and second prism members is a reflection type hologram. If the tilt angle of the surface of the holographic element is set at an angle different from 45 degrees with respect to the visual axis (i.e. the axial principal ray reaching the exit pupil from the surface of the viewing optical system closest to the exit pupil), the overall thickness of the viewing optical system can be reduced, and thus a compact and lightweight optical system can be realized. To correct decentration aberrations occurring owing to the arrangement in which an oblique mirror comprising the holographic element is set at an angle different from 45 degrees with respect to the visual axis, free-form surfaces are used as a surface through which light from the image display device enters the prism, a surface reflecting diffracted light from the reflection type holographic element, and a surface in front of the observer""s eye. Further, a power is given to the substrate surface configuration of the reflection type holographic element. Thus, coma and field curvature are corrected favorably.
That is, it is important to satisfy the following condition:
45xc2x0 less than xcex8 less than 85xc2x0xe2x80x83xe2x80x83(1)
where xcex8 is, as shown in FIG. 24, the angle between a tangential plane at a position A of intersection between the axial principal ray 2 and the substrate surface of the holographic element 6 and the axial principal ray 2 reaching the exit pupil 1 from the surface 41 of the viewing optical system closest to the exit pupil 1.
If the angle xcex8 is not larger than the lower limit of the condition (1), i.e. 45xc2x0, the tilt of the oblique mirror comprising the holographic element becomes excessively small. Consequently, the viewing optical system increases in thickness, resulting in a large and heavyweight optical system. If the angle xcex8 is not smaller than the upper limit, i.e. 85xc2x0, the amount of decentration of the viewing optical system becomes excessively large. Consequently, it is difficult to correct decentration aberrations. Thus, it becomes difficult to observe an image having a high contrast and favorably corrected for distortion.
It is more desirable to satisfy the following condition:
55xc2x0 less than xcex8 less than 80xc2x0xe2x80x83xe2x80x83(1-1)
The meaning of the lower and upper limits of the condition (1-1) is the same as that of the lower and upper limits of the condition (1).
It is even more desirable to satisfy the following condition:
65xc2x0 less than xcex8 less than 75xc2x0xe2x80x83xe2x80x83(1-2)
The meaning of the lower and upper limits of the condition (1-2) is the same as that of the lower and upper limits of the condition (1).
In the viewing optical system according to the present invention, the inside of the viewing optical system is filled with a medium, e.g. a glass or plastic material, in the form of the first and second prism members, thereby increasing the optical power based on the surface configuration of each optical functional surface, and thus favorably correcting aberrations, e.g. coma and field curvature.
Incidentally, the reflection type holographic element provided as an oblique mirror in the present invention is, in general, a film-type planar holographic element. It is desirable that the first exit surface of the first prism member and the second entrance surface of the second prism member, each of which serves as a substrate to which the planar holographic element is bonded, should have a plane surface configuration or a cylindrical surface configuration.
It is also possible to use a spherical surface or a toric surface as a substrate to which the planar holographic element is bonded. If the spherical or toric surface satisfies the following conditions, mass-production can be realized by using the planar holographic element:
xe2x88x922.0 less than Da/Ra less than 2.0xe2x80x83xe2x80x83(2)
xe2x88x920.05 less than Db/Rb less than 0.05xe2x80x83xe2x80x83(3)
where Ra and Da are a curvature radius and an outer diameter of the surface in the direction of an axis where the surface has a larger curvature, and Rb and Db are a curvature radius and an outer diameter of the surface in the direction of an axis where the surface has a smaller curvature.
In the conditions (2) and (3), the lower limits, i.e. xe2x88x922.0 and xe2x88x920.05, are limit values in a case where the holographic element is bonded to a concave surface. The upper limits, i.e. 2.0 and 0.05, are limit values in a case where the holographic element is bonded to a convex surface. If Da/Ra and Db/Rb are not within the ranges defined by the conditions (2) and (3), the planar holographic element wrinkles at the peripheral portion of the curved surface. Thus, it becomes difficult to bond the holographic element uniformly and hence impossible to obtain the desired optical performance of the holographic element.
It is more desirable to satisfy the following conditions:
xe2x88x922.0 less than Da/Ra less than 2.0xe2x80x83xe2x80x83(2-1)
0.02 less than Db/Rb less than 0.02xe2x80x83xe2x80x83(3-1)
The meaning of the lower and upper limits of the conditions (2-1) and (3-1) is the same as that of the lower and upper limits of the conditions (2) and (3).
It is even more desirable to satisfy the following conditions:
xe2x88x922.0 less than Da/Ra less than 2.0xe2x80x83xe2x80x83(2-2)
xe2x88x920.015 less than Db/Rb less than 0.015xe2x80x83xe2x80x83(3-2)
The meaning of the lower and upper limits of the conditions (2-2) and (3-2) is the same as that of the lower and upper limits of the conditions (2) and (3).
It is preferable in the viewing optical system according to the present invention that the second exit surface of the second prism member should have a rotationally asymmetric curved surface configuration that corrects at least either one of rotationally asymmetric coma and astigmatism produced in the ocular optical member.
Further, it is preferable in the viewing optical system according to the present invention that the rotationally asymmetric curved surface configuration of the second exit surface of the second prism member should be a free-form surface having only one plane of symmetry, and the plane of symmetry of the free-form surface should be coincident with a plane (YZ-plane) in which the optical axis is folded.
It is desirable in the present invention that the surfaces constituting the first prism member and those constituting the second prism member should be rotationally asymmetric surfaces, e.g. free-form surfaces, from the viewpoint of realizing an optical system capable of favorably correcting rotationally asymmetric distortion and exhibiting favorable telecentricity. However, those surfaces may be formed from rotationally symmetric surfaces, e.g. spherical surfaces, aspherical surfaces, or anamorphic surfaces.
In the viewing optical system according to the present invention, a light beam from an observation image formed by the observation image forming member is passed through the first entrance surface to enter the first prism member. The light beam entering the first prism member is made incident on a volume hologram at a first incident angle within the range of angle selectivity of the hologram. After being reflected and diffracted from the hologram, the light beam is reflected by the reflecting surface. The reflected light beam is incident on the volume hologram surface again at a second incident angle. At this time, because the second incident angle is not within the range of angle selectivity of the volume hologram, the diffraction efficiency is extremely low. Consequently, the incident light beam passes through the first exit surface substantially as it is, and enters the second prism member through the second entrance surface.
The light beam entering the second prism member exits the second prism member through the second exit surface as it is, and is then led to the observer""s eyeball.
In the viewing optical system according to the present invention, an optical member, e.g. a prism, a plane-parallel plate of glass, or a positive or negative lens, may be disposed between the first entrance surface of the first prism member and the observation image forming member.
Further, in the viewing optical system according to the present invention, an optical member, e.g. a prism, a plane-parallel plate of glass, or a positive or negative lens, may be disposed between the second exit surface of the second prism member and the exit pupil.
In the case of using an image display device, e.g. an LCD (Liquid Crystal Display), it is necessary in order to perform image display with high contrast to enlarge and display an image through an optical system with favorable telecentricity. The described arrangements of the optical system according to the present invention are applicable not only to a viewing optical system but also to an image pickup optical system. In the latter case, when an image pickup device, e.g. a CCD, is used, it is also important to pick up an image through an optical system with favorable telecentricity from the viewpoint of preventing shading or the like.
In the present invention, the eye relief is long relative to the focal length of the entire optical system. Therefore, the extra-axial principal rays are tilted with respect to the image display device in a direction in which the extra-axial light beams converge. To realize an optical system having enhanced telecentricity and an increased eye relief as well as a compact structure, it is desirable to place a negative power in the optical path near the image display device (image pickup device) and a positive power on the eye (object) side.
The above means that the optical system has an arrangement obtained by inverting a retrofocus type optical system. That is, it is important to give a negative power to an oblique mirror surface in the optical system near the image display device. To ensure telecentricity over the whole area of an image field with a difference in length between two orthogonal axis directions, it is particularly important that the oblique mirror surface should have a larger negative power in an axis direction of the aspect ratio in which the oblique mirror surface is longer than in the direction of the other axis.
Let us assume that the length in the X-axis direction of the image display device is Dx, and the length in the Y-axis direction of the image display device is Dy. That is, the aspect ratio of the image display device is denoted by Dx:Dy. Further, the curvature radius in the X-axis direction of the oblique mirror surface is assumed to be Rx, and the curvature radius in the Y-axis direction of the oblique mirror surface is assumed to be Ry. On these assumptions, if the following conditions are satisfied, telecentricity can be ensured to obtain favorable optical performance in a case where there is no displacement in the X-axis direction (this is true of all Examples described later):
xe2x88x921.0xe2x89xa6Dx/Rxxe2x89xa60xe2x80x83xe2x80x83(4)
xe2x88x921.0xe2x89xa6xe2x88x92Dy/Ryxe2x89xa60xe2x80x83xe2x80x83(5)
If the upper limits of the conditions (4) and (5), i.e. 0, are exceeded, the oblique mirror surface has a positive power. Consequently, the principal rays further tilt in the direction of convergence, and it becomes impossible to capture a high-contrast image from the image display device. If Dx/Rx and Dy/Ry are smaller than the lower limits of the conditions (4) and (5), i.e. xe2x88x921.0, the image display device is excessively large in size, or the negative power of the oblique mirror surface is excessively large. Consequently, the tilt angle of the principal rays becomes rather divergent. Accordingly, it becomes impossible to capture a high-contrast image from the image display device.
It is more desirable to satisfy the following conditions:
xe2x88x920.5xe2x89xa6Dx/Rxxe2x89xa6xe2x88x920xe2x80x83xe2x80x83(4-1)
xe2x88x920.5 less than xe2x88x92Dy/Ryxe2x89xa60xe2x80x83xe2x80x83(5-1)
The meaning of the lower and upper limits of the conditions (4-1) and (5-1) is the same as that of the lower and upper limits of the conditions (4) and (5).
It is even more desirable to satisfy the following conditions:
xe2x88x920.1xe2x89xa6Dx/Rxxe2x89xa60xe2x80x83xe2x80x83(4-2)
xe2x88x920.1xe2x89xa6Dy/Ryxe2x89xa60xe2x80x83xe2x80x83(5-2)
The meaning of the lower and upper limits of the conditions (4-2) and (5-2) is the same as that of the lower and upper limits of the conditions (4) and (5).
With the above-described arrangement, a holographic element is provided on an oblique mirror surface having a plane or cylindrical substrate surface configuration. Alternatively, a planar holographic element is provided on an oblique mirror surface having a spherical or toric substrate surface configuration satisfying the conditions (2) and (3) for preventing the planar holographic element from wrinkling at the peripheral portion thereof when bonded to the spherical or toric surface. This arrangement dispenses with the need to provide a half-mirror for branching the optical path or to provide an air space. Accordingly, it is possible to obtain a viewing optical system that allows observation of a bright displayed image favorably corrected for aberrations with minimal loss of light quantity and is easy to assemble, resistant to impact such as vibration, lightweight and compact and further permits a hologram to be bonded easily, and it is also possible to obtain an apparatus using the viewing optical system.
It should be noted that the described arrangements of the optical system according to the present invention are applicable not only to a viewing system but also to an image pickup system.
The image pickup optical system according to the present invention has an image pickup device placed in an image plane to pick up an image of an object, an aperture stop placed in a pupil plane to reduce the brightness of a light beam from the object, and an image-forming optical member disposed between the image plane and the pupil plane to lead the object image to the image plane. The image-forming optical member includes at least a second prism member and a first prism member. The second prism member has, at least, a third entrance surface through which light rays emanating from the object and passing through the aperture stop enter the second prism member, and a third exit surface through which the light rays exit the second prism member. The third entrance surface and the third exit surface are disposed to face each other across a second prism medium. The first prism member has, at least, a fourth entrance surface through which the light rays exiting from the second prism member enter the first prism member, a reflecting surface reflecting the light rays within the first prism member, and a fourth exit surface through which the light rays exit the first prism member. The fourth entrance surface, the reflecting surface and the fourth exit surface are disposed to face each other across a first prism medium. The second prism member and the first prism member are cemented together with a holographic element interposed between the third exit surface and the fourth entrance surface. The reflecting surface of the first prism member is a concave surface that gives a positive power to the light rays when reflecting them.
The third exit surface and the fourth entrance surface are each formed from a plane surface or a cylindrical surface.
Alternatively, the third exit surface and the fourth entrance surface are each formed from a spherical surface or a toric surface satisfying the following conditions:
xe2x88x922.0 less than Da/Ra less than 2.0xe2x80x83xe2x80x83(2)
xe2x88x920.05 less than Db/Rb less than 0.05xe2x80x83xe2x80x83(3)
where Ra and Da are a curvature radius and an outer diameter of the surface in the direction of an axis where the surface has a larger curvature, and Rb and Db are a curvature radius and an outer diameter of the surface in the direction of an axis where the surface has a smaller curvature.
That is, the image pickup optical system according to the present invention is formed by replacing the observation image forming member, the exit pupil and the ocular optical member in the viewing optical system according to the present invention with the image pickup device, the aperture stop and the image-forming optical member, respectively.
It is also preferable in the image pickup optical system to adopt arrangements similar to those of the viewing optical system, e.g. the above-described conditional expressions.
Further, in the viewing optical system according to the present invention, the reflecting surface of the first prism member should preferably be formed by mirror coating.
The reflecting surface of the first prism member may be arranged in the form of a totally reflecting surface that reflects a light beam incident thereon at an angle exceeding the total reflection critical angle but transmits a light beam incident thereon at an angle not exceeding the total reflection critical angle. It is also possible to provide a light-transmitting optical member on the reflecting surface side of the first prism member.
With this arrangement, see-through observation can be performed. Accordingly, the user can continue wearing a head- or face-mounted image display apparatus using the viewing optical system according to the present invention without interference with the normal observation of the outside. Thus, it is possible to save time and effort to put on or take off the head- or face-mounted image display apparatus.
It is also possible to view both an external observation image and an image from the image display device as a superimposed multiple image.
It should be noted that the reflecting surface of the first prism member may be formed from a half-mirror to allow see-through observation.
It is also possible to construct a head-mounted image display apparatus having a body unit containing an image display device and any of the foregoing viewing optical systems according to the present invention arranged as an ocular optical system. The head-mounted image display apparatus further has a support member for supporting the body unit on the head of an observer in such a manner that the exit pupil of the viewing optical system is held at the position of an eyeball of the observer, and a speaker member for giving voice to an ear of the observer.
The above-described head-mounted image display apparatus may be arranged such that the body unit has a viewing optical system for a right eye and a viewing optical system for a left eye, and the speaker member has a speaker member for a right ear and a speaker member for a left ear.
In the head-mounted image display apparatus, the speaker member may be an earphone.
In the viewing optical system according to the present invention, a light ray from the object center that passes through the center of the pupil and reaches the center of the image plane in backward ray tracing is defined as an axial principal ray. In the image pickup optical system according to the present invention, a light rays from the object center that passes through the center of the aperture stop and reaches the center of the image plane in forward ray tracing is defined as an axial principal ray. In the optical system according to the present invention, if at least one reflecting surface is not decentered with respect to the axial principal ray, the axial principal ray travels along the same optical path when incident on and reflected from the reflecting surface, and thus the axial principal ray is intercepted in the optical system undesirably. As a result, an image is formed from only a light beam whose central portion is shaded. Consequently, the center of the image is unfavorably dark, or no image is formed in the center of the image field. For this reason, a decentered prism is used as each prism member in the present invention.
When a reflecting surface with a power is decentered with respect to the axial principal ray, it is desirable that at least one of the surfaces constituting each prism member used in the present invention should be a rotationally asymmetric surface. It is particularly preferable from the viewpoint of correcting aberrations that at least one reflecting surface of the prism members should be a rotationally asymmetric surface.
To use an optical path in a common region repeatedly by folding the optical path, the optical system has to be decentered. However, if the optical system is formed into a decentered optical system in order to fold the optical path, decentration aberrations such as rotationally asymmetric distortion and rotationally asymmetric field curvature occur. To correct the decentration aberrations, a rotationally asymmetric surface is used as stated above.
The rotationally asymmetric surface used in the present invention can be formed from an anamorphic surface, a toric surface, or a plane-symmetry free-form surface having only one plane of symmetry. It is preferable to use a free-form surface having only one plane of symmetry as a rotationally asymmetric surface.
In the present invention, the axial principal ray is defined as follows. In the viewing optical system, a light ray passing through the center of the exit pupil and reaching the center of the observation image forming member in the backward ray tracing is defined as an axial principal ray. In the image pickup optical system, a light ray passing through the center of the aperture stop and reaching the center of the image pickup device in the forward ray tracing is defined as an axial principal ray. An optical axis defined by a straight line along which the axial principal ray travels from the center of the exit pupil or the aperture stop until it intersects the second exit surface of the second prism member is defined as a Z-axis. An axis perpendicularly intersecting the Z-axis in the decentration plane of each surface constituting the second prism member is defined as a Y-axis. An axis perpendicularly intersecting the Z-axis and also perpendicularly intersecting the Y-axis is defined as an X-axis. Further, the center of the exit pupil or the aperture stop is defined as the origin of the coordinate system in the viewing optical system or the image pickup optical system according to the present invention. Further, in the present invention, the surface Nos. are put in the order of backward ray tracing from the exit pupil toward the observation image forming member or in the order of forward ray tracing from the aperture stop toward the image pickup device, as stated above. The direction along which the axial principal ray from the exit pupil reaches the observation image forming member or the direction along which the axial principal ray from the aperture stop reaches the image pickup device is defined as a positive direction of the Z-axis. The direction in which the Y-axis extends toward the observation image forming member or the direction in which the Y-axis extends toward the image pickup device is defined as a positive direction of the Y-axis. The direction in which the X-axis constitutes a right-handed system in combination with the Y- and Z-axes is defined as a positive direction of the X-axis.
Free-form surfaces used in the present invention are defined by the following equation (a). The Z-axis of the defining equation is the axis of a free-form surface.                     Z        =                              c            ⁢                          xe2x80x83                        ⁢                                          r                2                            /                              [                                  1                  +                                                                                    xe2x80x83                                                              ⁢                                          {                                              1                        -                                                                              (                                                          1                              +                              k                                                        )                                                    ⁢                                                      c                            2                                                    ⁢                                                      r                            2                                                                                              }                                                                      ]                                              +                                    ∑                              j                =                2                            ∞                        ⁢                          xe2x80x83                        ⁢                                          C                j                            ⁢                              X                m                            ⁢                              Y                n                                                                        (        a        )            
In Eq. (a), the first term is a spherical surface term, and the second term is a free-form surface term.
In the spherical surface term:
c: the curvature at the vertex
k: a conic constant
r=√ (X2+Y2)
The free-form surface term is given by                                           ∑                          j              =              2                        ∞                    ⁢                      xe2x80x83                    ⁢                                    C              j                        ⁢                          X              m                        ⁢                          Y              n                                      =                ⁢                                            C              2                        ⁢            X                    +                                    C              3                        ⁢            Y                    +                                    C              4                        ⁢                          X              2                                +                                    C              5                        ⁢            X            ⁢                          xe2x80x83                        ⁢            Y                    +                                    C              6                        ⁢                          Y              2                                +                                                ⁢                                            C              7                        ⁢                          X              3                                +                                    C              8                        ⁢                          X              2                        ⁢            Y                    +                                    C              9                        ⁢            X            ⁢                          xe2x80x83                        ⁢                          Y              2                                +                                    C              10                        ⁢                          X              3                                +                                    C              11                        ⁢                          X              4                                +                                                ⁢                                            C              12                        ⁢                          X              3                        ⁢            Y                    +                                    C              13                        ⁢                          X              2                        ⁢                          xe2x80x83                        ⁢                          Y              2                                +                                    C              14                        ⁢            X            ⁢                          xe2x80x83                        ⁢                          Y              3                                +                                    C              15                        ⁢                          Y              4                                +                                    C              16                        ⁢                          X              5                                +                                                ⁢                                            C              17                        ⁢                          X              4                        ⁢            Y                    +                                    C              18                        ⁢                          X              3                        ⁢                          xe2x80x83                        ⁢                          Y              2                                +                                    C              19                        ⁢                          X              2                        ⁢                          xe2x80x83                        ⁢                          Y              3                                +                                    C              20                        ⁢            X            ⁢                          xe2x80x83                        ⁢                          Y              4                                +                                                ⁢                                            C              21                        ⁢                          Y              5                                +                                    C              22                        ⁢                          X              6                                +                                    C              23                        ⁢                          X              5                        ⁢            Y                    +                                    C              24                        ⁢                          X              4                        ⁢                          xe2x80x83                        ⁢                          Y              2                                +                                    C              25                        ⁢                          X              3                        ⁢                          xe2x80x83                        ⁢                          Y              3                                +                                                ⁢                                            C              26                        ⁢                          X              2                        ⁢                          xe2x80x83                        ⁢                          Y              4                                +                                    C              27                        ⁢            X            ⁢                          xe2x80x83                        ⁢                          Y              5                                +                                    C              28                        ⁢                          Y              6                                +                                    C              29                        ⁢                          X              7                                +                                    C              30                        ⁢                          X              6                        ⁢            Y                    +                                                ⁢                                            C              31                        ⁢                          X              5                        ⁢                          xe2x80x83                        ⁢                          Y              2                                +                                    C              32                        ⁢                          X              4                        ⁢                          xe2x80x83                        ⁢                          Y              3                                +                                    C              33                        ⁢                          X              3                        ⁢                          xe2x80x83                        ⁢                          Y              4                                +                                    C              34                        ⁢                          X              2                        ⁢                          xe2x80x83                        ⁢                          Y              5                                +                                                ⁢                                            C              35                        ⁢            X            ⁢                          xe2x80x83                        ⁢                          Y              6                                +                                    C              36                        ⁢                          Y              7                        ⁢                          xe2x80x83                        ⁢            …                          ⁢                  xe2x80x83                    
where Cj (j is an integer of 2 or higher) are coefficients, and j={(m+n)2+m+3n}/2+1 (m and n are integers of zero or higher).
In general, the above-described free-form surface does not have planes of symmetry in both the XZ- and YZ-planes. However, a free-form surface having only one plane of symmetry parallel to the YZ-plane is obtained by making all terms of odd-numbered degrees with respect to X zero. For example, in the above defining equation (a), the coefficients of the terms C2, C5, C7, C9, C12, C14, C16, C18, C20, C23, C25, C27, C29, C31, C33, C35, . . . are set equal to zero. By doing so, it is possible to obtain a free-form surface having only one plane of symmetry parallel to the YZ-plane.
A free-form surface having only one plane of symmetry parallel to the XZ-plane is obtained by making all terms of odd-numbered degrees with respect to Y zero. For example, in the above defining equation (a), the coefficients of the terms C3, C5, C8, C10, C12, C14, C17, C19, C21, C23, C25, C27, C30, C32, C34, C36, . . . are set equal to zero. By doing so, it is possible to obtain a free-form surface having only one plane of symmetry parallel to the XZ-plane.
Furthermore, the direction of decentration is determined in correspondence to either of the directions of the above-described planes of symmetry. For example, with respect to the plane of symmetry parallel to the YZ-plane, the direction of decentration of the optical system is determined to be the Y-axis direction. With respect to the plane of symmetry parallel to the XZ-plane, the direction of decentration of the optical system is determined to be the X-axis direction. By doing so, rotationally asymmetric aberrations due to decentration can be corrected effectively, and at the same time, productivity can be improved.
It should be noted that the above defining equation (a) is shown as merely an example, and that the feature of the present invention resides in that rotationally asymmetric aberrations due to decentration are corrected and, at the same time, productivity is improved by using a rotationally asymmetric surface having only one plane of symmetry. Therefore, the same advantageous effect can be obtained for any defining equation other than the above defining equation (a) that expresses such a rotationally asymmetric surface.
In the present invention, the reflecting surface provided in the prism member may be a plane-symmetry free-form surface having only one plane of symmetry.
The configuration of an anamorphic surface is defined by the following equation (b). A straight line passing through the origin of the surface configuration perpendicularly to the optical surface is the axis of the anamorphic surface.
Z=(Cxxc2x7X2+Cyxc2x7Y2)/1+{1xe2x88x92(1+Kx)Cx2xc2x7X2xe2x88x92(1+Ky)Cy2xc2x7Y2}1/2+xcexa3Rn{(1xe2x88x92Pn)X2+(1+Pn)Y2}(n+1)xe2x80x83xe2x80x83(b)
Assuming that n=4 (polynomial of degree 4), for example, an anamorphic surface may be expressed by an expanded form of the above equation (b) as follows:                                                         Z              =                            ⁢                                                (                                                            C                      ⁢                                              xe2x80x83                                            ⁢                                              x                        ·                                                  X                          2                                                                                      +                                          C                      ⁢                                              xe2x80x83                                            ⁢                                              y                        ·                                                  Y                          2                                                                                                      )                                /                                  [                                      1                    +                                          {                                              1                        -                                                                              (                                                          1                              +                                                              K                                ⁢                                                                  xe2x80x83                                                                ⁢                                x                                                                                      )                                                    ⁢                          C                          ⁢                                                      xe2x80x83                                                    ⁢                                                                                    x                              2                                                        ·                                                          X                              2                                                                                                      -                                                                                                                                                                                                                                              ⁢                                                                  (                                                  1                          +                                                      K                            ⁢                                                          xe2x80x83                                                        ⁢                            y                                                                          )                                            ⁢                      C                      ⁢                                              xe2x80x83                                            ⁢                                                                        y                          2                                                ·                                                  Y                          2                                                                                      }                                                        1                    /                    2                                                  ]                            +                              R1                ⁢                                                      {                                                                                            (                                                      1                            -                            P1                                                    )                                                ⁢                                                  X                          2                                                                    +                                                                        (                                                      1                            +                            P1                                                    )                                                ⁢                                                  Y                          2                                                                                      }                                    2                                            +                                                                                        ⁢                                                R2                  ⁢                                                            {                                                                                                    (                                                          1                              -                              P2                                                        )                                                    ⁢                                                      X                            2                                                                          +                                                                              (                                                          1                              +                              P2                                                        )                                                    ⁢                                                      Y                            2                                                                                              }                                        3                                                  +                                  R3                  ⁢                                                            {                                                                                                    (                                                          1                              -                              P3                                                        )                                                    ⁢                                                      X                            2                                                                          +                                                                              (                                                          1                              +                              P3                                                        )                                                    ⁢                                                      Y                            2                                                                                              }                                        4                                                  +                                                                                                      ⁢                              R4                ⁢                                                      {                                                                                            (                                                      1                            -                            P4                                                    )                                                ⁢                                                  X                          2                                                                    +                                                                        (                                                      1                            +                            P4                                                    )                                                ⁢                                                  Y                          2                                                                                      }                                    5                                                                                        (        c        )            
where Z is the amount of deviation from a plane tangent to the origin of the surface configuration; Cx is the curvature in the X-axis direction; Cy is the curvature in the Y-axis direction; Kx is the conic coefficient in the X-axis direction; Ky is the conic coefficient in the Y-axis direction; Rn is the rotationally symmetric component of the spherical surface term; and Pn is the rotationally asymmetric component of the aspherical surface term. It should be noted that the radius of curvature Rx in the X-axis direction and the radius of curvature Ry in the Y-axis direction are related to the curvatures Cx and Cy as follows:
Rx=1/Cx, Ry=1/Cy
Toric surfaces include an X-toric surface and a Y-toric surface, which are defined by the following equations (d) and (e), respectively. A straight line passing through the origin of the surface configuration perpendicularly to the optical surface is the axis of the toric surface. The X-toric surface is given by
F(X)=Cxxc2x7X2/[1+{1xe2x88x92(1+K)Cx2xc2x7X2}1/2]+AX4+BX6+CX8+DX10
Z=F(X)+(xc2xd)Cy{Y2+Z2xe2x88x92F(X)2}xe2x80x83xe2x80x83(d)
The Y-toric surface is given by
F(Y)=Cyxc2x7Y2/[1+{1xe2x88x92(1+K)Cy2xc2x7Y2}1/2]+AY4+BY6+CY8+DY10
Z=F(Y)+(xc2xd)Cx{X2+Z2xe2x88x92F(Y)2}xe2x80x83xe2x80x83(e)
In the above equations, Z is the amount of deviation from a plane tangent to the origin of the surface configuration; Cx is the curvature in the X-axis direction; Cy is the curvature in the Y-axis direction; K is a conic coefficient; and A, B, C and D are aspherical coefficients, respectively. It should be noted that the radius of curvature Rx in the X-axis direction and the radius of curvature Ry in the Y-axis direction are related to the curvatures Cx and Cy as follows:
xe2x80x83Rx=1/Cx, Ry=1/Cy
Holographic elements include two different types, i.e. relief holograms, and volume holograms. Relief holograms have the nature that the incident angle selectivity and wavelength selectivity are low, and they diffract light of specific wavelength incident thereon at a specific angle to form an image by the desired order of light. However, the relief holograms also diffract light of other wavelengths incident thereon at other angles as unwanted orders of light to form an undesired image. On the other hand, the volume holograms have the nature that the incident angle selectivity and wavelength selectivity are high, and hence they diffract only light of specific wavelength incident thereon at a specific angle to form an image by the desired order of light. The volume holograms transmit substantially all the other orders of light as zero-order light and are therefore unlikely to form an undesired image by unwanted orders of light.
Therefore, if a reflection type volume hologram is used as a holographic element in the present invention, it is possible to prevent image blur due to unwanted orders of diffracted light and hence possible to obtain a clear observation image.
It should be noted that a volume hologram (HOE) used as a holographic element in the present invention is defined as follows. FIG. 25 is a principle diagram for defining the HOE in the present invention.
First, tracing of rays of wavelength xcex incident on and exiting from the HOE surface is given by the following equation (f) using the optical path difference function "PHgr"0 on the HOE surface defined with respect to the reference wavelength xcex0=HWL:
ndQdxc3x97N=niQixc3x97N+m(xcex/xcex0)∇"PHgr"0xc3x97Nxe2x80x83xe2x80x83(f)
where N is the normal vector to the HOE surface; ni (nd) is the refractive index on the incidence side (exit side); and Qi (Qd) is the incidence (exit) vector (unit vector). In addition, m=HOR is the order of diffraction of emergent light.
Assuming that the HOE is produced (defined) by two point sources of reference wavelength xcex0, that is, as shown in FIG. 25, by interference between object light from a light source at point P1=(HX1, HY1, HZ1) and reference light from a light source at point P2=(HX2, HY2, HZ2),
"PHgr"0="PHgr"02P=n2xc2x7s2xc2x7r2xe2x88x92n1xc2x7s1xc2x7r1
where r1 (r2) is the distance ( greater than 0) from the point P1 (point P2) to a predetermined coordinate point P on the HOE surface; n1 (n2) is the refractive index of a medium in which the HOE is placed during the production (definition) on the side where the point P1 (point P2) is located; and s1=HV1 and s2=HV2 are signs to consider the direction of travel of light. The sign is REA=+1 when the light source is a divergent light source (real point source). Conversely, when the light source is a convergent light source (virtual point source), the sign is VIR=xe2x88x921. Regarding the definition of the HOE in lens data, the refractive index n1 (n2) of a medium in which the HOE is placed during the production (definition) is the refractive index of a medium with which the HOE surface is in contact on the side where the point P1 (P2) is present in the lens data.
In a general case, reference light and object light used to produce an HOE are not always spherical wave. The optical path difference function "PHgr"0 of the HOE in this case may be expressed by adding a polynomially-expressed additive phase term "PHgr"0Poly (optical path difference function at the reference wavelength xcex0) as follows:
"PHgr"0="PHgr"02P+"PHgr"0Polyxe2x80x83xe2x80x83(g)
In the above equation (g), the polynomial expression is as follows:                               Φ          0          poly                =                ⁢                              ∑            j                    ⁢                                    H              j                        ·                          x              m                        ·                          y              n                                                              =                ⁢                                            H              1                        ⁢            x                    +                                    H              2                        ⁢            y                    +                                    H              3                        ⁢                          x              2                                +                                    H              4                        ⁢            x            ⁢                          xe2x80x83                        ⁢            y                    +                                    H              5                        ⁢                          y              2                                +                                                ⁢                                            H              6                        ⁢                          x              3                                +                                    H              7                        ⁢                          x              2                        ⁢            y                    +                                    H              8                        ⁢            x            ⁢                          xe2x80x83                        ⁢                          y              2                                +                                    H              9                        ⁢                          y              3                                +          …                    
In general, it may be defined as follows:
j={(m+n)2+m+3n}/2
In the above expression, Hj is the coefficient of each term.
For the convenience of optical design, the optical path difference function "PHgr"0 may be expressed by only the additive term to define the HOE as follows:
"PHgr"0="PHgr"0Poly
For example, if the two point sources P1 (P2) are made coincident with each other, the interference component xcfx8602P of the optical path difference function "PHgr"0 is zero. This is equivalent to expressing the optical path difference function substantially only by the additive term (polynomial expression).
It should be noted that the foregoing description of the HOE has been made all with regard to local coordinates based on the HOE origin.
Examples of constituent parameters defining an HOE are as follows:
The following is a description of the principle of reflection, diffraction and transmission at the surface of a volume hologram used in the present invention. Regarding the simulation of diffraction efficiency, let us show the simulation of diffraction efficiency for s-polarized light component based on Kogelnik""s theory. The simulation was performed on Example 1 (described later). However, this is true of the other examples.
In this example, LED light sources having center wavelengths of 630 nm, 520 nm and 470 nm, respectively, were used as light sources for R, G and B bands, together with narrow-band filters to narrow the bandwidth to about xc2x15 nm to 10 nm in center wavelength. Let us show the results of calculation of the diffraction efficiency at a volume hologram surface for the axial principal ray in the G band, by way of example. It should be noted that the calculation results were obtained under the conditions that the reference refractive index was 1.5, the refractive index modulation was 0.05, and the thickness of the holographic element was 10 xcexcm. The diffraction efficiency when the angle of incidence of the axial principal ray on the volume hologram surface was 47.3xc2x0 and the reflection diffraction angle was 46.9xc2x0 is shown in FIGS. 26 and 27. FIG. 26 is a graph showing the diffraction efficiency (ordinate axis) with respect to the incident angle (abscissa axis) of the axial principal ray of wavelength 520 nm. FIG. 27 is a graph showing the diffraction efficiency (ordinate axis) for the axial principal ray incident at an angle of 47.3xc2x0 with respect to wavelength (abscissa axis).
It will be understood from FIG. 26 that a high diffraction efficiency, i.e. approximately 100%, can be obtained at an incident angle in the neighborhood of 47.3xc2x0. From FIG. 27, it will be understood that a favorable reflection and diffraction efficiency can be obtained in a wavelength range of 520 nmxc2x120 nm. Meanwhile, light rays reflected from the reflecting surface of the first prism member after being reflected and diffracted from the volume hologram surface are incident on the hologram surface again. At this time, the axial principal ray is incident on the hologram surface at an angle of 18.7xc2x0. It will be understood from FIG. 26 that the incident angle of 18.7xc2x0 is not within the angle selectivity range of the volume holographic element, in which it exhibits a high diffraction efficiency, and the diffraction efficiency is as low as about 0%. Therefore, the light rays pass through the volume holographic element as they are.
The above discussion is true of the R band and the B band. It is also possible to use a switching holographic element employing a liquid crystal as the above-described holographic element.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.